Need to extract into a test subgroup to make sure we only skip the
tests that need display support.

v2: Chris pointed out that "read-all-entries" was the original non-kms
tests, and we don't want to skip that if there's no output. Make a
seperate test for this.

Also, that kind of where libraries magically second-guess what the
test might have wanted when it supplies an invalid request is exactly
why I want to fix the igt_display_init API regression.

v3: Actually squash in the hunk that was supposed to do v2 into this
patch (Antonio).

Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-By: Antonio Argenziano <antonio.argenziano@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>